1. Field of the Invention
The present invention relates generally to an LED driver circuit, and more particularly to a constant current driver circuit that can drive LEDs and provide desired luminance.
2. Description of Prior Art
Conventional LED driver circuit drives the LED, and comprises an AC input and a full bridge rectifier. Please refer to FIG. 1A, which illustrates a schematic diagram of a prior art driver circuit. The full bridge rectifier includes diodes D11, D21, D31, and D41. The AC input to the circuit in FIG. 1 is indicated by the symbol for an AC source 10. The AC source 10 is used for generating an alternating voltage. The AC to DC rectification is performed by use of the full bridge rectifier. The diodes D31 and D21 conduct the positive half period current, and the diodes D41 and D11 conduct the negative half period current.
A current limit resistor 45 can be added to the full bridge rectifier. The current limit resistor 45 can limit the current flowing through the diodes D31 and D21 when conducting the positive half period current, and limit the current flowing through the diodes D41 and D11 when conducting the negative half period current. The diodes can be protected from over current damage by the current limit resistor 45.
However, the prior art driver circuit has one major drawback. FIG. 1B illustrates a current waveform that flows through the current limit resistor 45. The value of the current I is equal to the voltage divided by the resistance R of the current limit resistor 45. The peak value Ipeak is the maximum current that the LEDs can withstand. The resistance R of the current limit resistor 45 is selected on consideration of the maximum voltage Vpeak. While the alternating voltage generated by the AC source 10 is subject to VRMS, VRMS times the square root of 2 gives the peak value Vpeak. However, VRMS varies slightly under the influence of household power consumption, and the diodes are often operated under insufficient current supplied conditions (less than maximum voltage Vpeak divided by the resistance R of the current limit resistor 45). Consequently, it is difficult to control the brightness of the driven LEDs accurately.